Electrical cables are widely used in telecommunications applications for the transmission of voice, video and data signals. Electrical cables typically include a conductive cable core surrounded by a jacket that provides mechanical strength and protection to the cable core. PVC is commonly used as a cable insulating and jacketing material since it is cheap and, with the addition of various elastomers, can be made extremely flexible, even at lower temperatures.
However, when PVC burns it produces considerable amounts of smoke and releases toxic halogen compounds, which account for many fire-related deaths. To reduce the risk of fire propagating through a building's ductwork, safety codes often require that plenum-rated cables meet industry standards for low smoke generation and low flame spread. Cables obtain the plenum rating upon successfully passing NFPA 262 (UL 910) flame propagation and smoke generation tests, which require that the materials used in conductor insulations and cable jackets be capable of withstanding a specified amount of heat for a specified amount of time without combustion or contributing significantly to the sustenance of a fire.
To successfully achieve a plenum rating, cables are constructed of materials that are more fire resistant and produce less smoke than traditional jacket materials. While there are several versions of PVC with varying characteristics, to Applicants' knowledge none are able to pass the plenum test. Some versions of PVC and polyolefins may attain plenum capability when combined with certain other polymers that are more fire resistant. However, maintaining the safety margins against the plenum flame test is sometimes difficult. Construction must be highly controlled and, in some instances, cable designs that pass the test one time may not pass on another trial.
More successful methods for increasing flame resistance include adding halogens to the jacket material. Fluoropolymers are commonly used to increase the fire resistance of the material. The most common thermoplastic polymer in plenum cables is fluorinated ethylene-1-propylene copolymer (FEP). See, for example, U.S. Pat. Nos. 5,841,072, 5,841,073, and 5,563,377, the disclosures of which are incorporated herein by reference.
Unfortunately, fluoropolymers are much more expensive to manufacture, thus the higher cost of plenum rated cables. Furthermore, fluoropolymers are tougher and more difficult to extrude, resulting in plenum cables that are not as flexible as PVC cables. Some cables include a composite of FEP and other materials. See, for example, U.S. Pat. No. 5,932,847, the disclosure of which is incorporated herein by reference. However, these composite designs often require twist length or expansion consideration to minimize signal propagation delay skew, as well as increase manufacturing complexity and product cost.
There is also a high concern about the true safety of halogen-based cables. When halogen-based cables burn (at whatever level they produce smoke), the smoke is corrosive and contains poisonous gases. Halogen-free polymer materials require complicated self-extinguishing formulations of compounds in order to obtain low smoke cable products. These materials add cost, complexity and may degrade the electrical performance of the cable.